Article

Causes and consequences of marine mammal population declines in southwest Alaska: A food-web perspective

Department of Ecology and Evolutionary Biology, University of California, Santa Cruz, CA 95060, USA.
Philosophical Transactions of The Royal Society B Biological Sciences (Impact Factor: 7.06). 07/2009; 364(1524):1647-58. DOI: 10.1098/rstb.2008.0231
Source: PubMed

ABSTRACT

Populations of sea otters, seals and sea lions have collapsed across much of southwest Alaska over the past several decades. The sea otter decline set off a trophic cascade in which the coastal marine ecosystem underwent a phase shift from kelp forests to deforested sea urchin barrens. This interaction in turn affected the distribution, abundance and productivity of numerous other species. Ecological consequences of the pinniped declines are largely unknown. Increased predation by transient (marine mammal-eating) killer whales probably caused the sea otter declines and may have caused the pinniped declines as well. Springer et al. proposed that killer whales, which purportedly fed extensively on great whales, expanded their diets to include a higher percentage of sea otters and pinnipeds following a sharp reduction in great whale numbers from post World War II industrial whaling. Critics of this hypothesis claim that great whales are not now and probably never were an important nutritional resource for killer whales. We used demographic/energetic analyses to evaluate whether or not a predator-prey system involving killer whales and the smaller marine mammals would be sustainable without some nutritional contribution from the great whales. Our results indicate that while such a system is possible, it could only exist under a narrow range of extreme conditions and is therefore highly unlikely.

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Available from: Terrie M Williams, Oct 29, 2014
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    • "megafaunal collapse hypothesis; Springer et al. 2003, Estes et al. 2009). Hence, considerable uncertainties typically accompany established paradigms that synthesize and shape our understanding of population declines, making it important to revisit and reevaluate these paradigms as new information becomes available and existing data sets are improved. "
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    ABSTRACT: https://albaylis.files.wordpress.com/2015/10/baylis_ecology-2015.pdf Considerable uncertainties often surround the causes of long-term changes in population abundance. One striking example is the precipitous decline of southern sea lions (SSL) (Otaria flavescens) at the Falkland Islands - from 80,555 pups in the mid 1930s to just 5,506 pups in 1965. Despite an increase in SSL abundance over the past two decades, the number of pups born in 2014 (minimum 4,443 pups) is less than 6% of the 1930s estimate. The order-of-magnitude decline is primarily attributed to commercial sealing in Argentina. Here, we test this established paradigm and alternative hypotheses by assessing (1) commercial sealing at the Falkland Islands, (2) winter migration of SSL from the Falkland Islands to Argentina, (3) whether the number of SSL in Argentina could have sustained the reported level of exploitation and, (4) environmental change. The most parsimonious hypothesis explaining the SSL population decline was environmental change. Specifically, analysis of 160 years of winter sea surface temperatures revealed marked changes, including a period of warming between 1930 and 1950 that was consistent with the period of SSL decline. Sea surface temperature changes likely influenced the distribution or availability of SSL prey and impacted its population dynamics. We suggest that historical harvesting may not always be the “smoking gun” as is often purported. Rather, our conclusions support the growing evidence for bottom-up forcing on the abundance of species at lower trophic levels (e.g., plankton and fish) and resulting impacts on higher trophic levels across a broad range of systems
    Full-text · Article · Oct 2015 · Ecology
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    • "megafaunal collapse hypothesis; Springer et al. 2003, Estes et al. 2009). Hence, considerable uncertainties typically accompany established paradigms that synthesize and shape our understanding of population declines, making it important to revisit and reevaluate these paradigms as new information becomes available and existing data sets are improved. "
    [Show abstract] [Hide abstract]
    ABSTRACT: https://albaylis.files.wordpress.com/2015/10/baylis_ecology-2015.pdf Considerable uncertainties often surround the causes of long-term changes in population abundance. One striking example is the precipitous decline of southern sea lions (SSL) (Otaria flavescens) at the Falkland Islands - from 80,555 pups in the mid 1930s to just 5,506 pups in 1965. Despite an increase in SSL abundance over the past two decades, the number of pups born in 2014 (minimum 4,443 pups) is less than 6% of the 1930s estimate. The order-of-magnitude decline is primarily attributed to commercial sealing in Argentina. Here, we test this established paradigm and alternative hypotheses by assessing (1) commercial sealing at the Falkland Islands, (2) winter migration of SSL from the Falkland Islands to Argentina, (3) whether the number of SSL in Argentina could have sustained the reported level of exploitation and, (4) environmental change. The most parsimonious hypothesis explaining the SSL population decline was environmental change. Specifically, analysis of 160 years of winter sea surface temperatures revealed marked changes, including a period of warming between 1930 and 1950 that was consistent with the period of SSL decline. Sea surface temperature changes likely influenced the distribution or availability of SSL prey and impacted its population dynamics. We suggest that historical harvesting may not always be the “smoking gun” as is often purported. Rather, our conclusions support the growing evidence for bottom-up forcing on the abundance of species at lower trophic levels (e.g., plankton and fish) and resulting impacts on higher trophic levels across a broad range of systems.
    Full-text · Article · Oct 2015 · Ecology
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    • "Open rectangles (n = 11) indicate historically used foraging locations, areas where otters were regularly seen but have not been observed since the early 1990s evaluated invertebrate prey using three 20-m transects and fish prey were evaluated using one 50-m × 2-m × 2-m transect oriented at random compass headings and secured to a haphazardly placed anchor at each site. Our target sampling depth contour was 5–15 m based on mean sea otter foraging depths (Bodkin et al. 2004) and the observation that sea otter prey distribution patterns in the Aleutians are generally similar across this depth range (Estes et al. 2009). We sampled invertebrate prey using three 0.25-m 2 quadrats placed randomly along each transect (n = 9 quadrats per site), inside which we counted all potential invertebrate prey, identified them to species, and measured them to the nearest millimeter (test or shell maximum linear length). "
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    ABSTRACT: Sea otters (Enhydra lutris) inhabiting the Aleutian Islands have stabilized at low abundance levels following a decline and currently exhibit restricted habitat-utilization patterns. Possible explanations for restricted habitat use by sea otters can be classified into two fundamentally different processes, bottom-up and top-down forcing. Bottom-up hypotheses argue that changes in the availability or nutritional quality of prey resources have led to the selective use of habitats that support the highest quality prey. In contrast, top-down hypotheses argue that increases in predation pressure from killer whales have led to the selective use of habitats that provide the most effective refuge from killer whale predation. A third hypothesis suggests that current restricted habitat use is based on a need for protection from storms. We tested all three hypotheses for restricted habitat use by comparing currently used and historically used sea otter foraging locations for: (1) prey availability and quality, (2) structural habitat complexity, and (3) exposure to prevailing storms. Our findings suggest that current use is based on physical habitat complexity and not on prey availability, prey quality, or protection from storms, providing further evidence for killer whale predation as a cause for restricted sea otter habitat use in the Aleutian Islands.
    Full-text · Article · Nov 2014 · Oecologia
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